The immune response results from the interactions of lymphocytes of many specificities and functions. One of the most intriguing subpopulations of lymphocytes is the helper T cell, whose activity is required for the optimal expression of humoral responses by B cells and cellular responses by other subclasses of T cells. The experiments are designed to further our understanding of helper T cell function and the mechanisms that control their activities. Specifically, we will: (1)\evaluate the role of helper T cells in responses of cytotoxic T lymphocytes, concentrating on a comparison of the requirements for interleukin 2 (IL-2) and other helper T cell-derived factors by resting and activated cytotoxic T cells; (2)\evaluate the requirements for activating B cells from a resting state to a state in which they are receptive to antigen nonspecific helper T cell replacing factors; and (3)\compare helper T cells and T cell-derived factors that interact with cytotoxic T cell precursors with those that interact with B cells. In all of these studies, we will use functionally defined clones of carrier-specific helper T cells and helper T cell lymphomas and hybridomas with defined functions and, in studies involving B cells, purified populations of hapten-binding B cells. These studies will provide information necessary to understand the basis of specificity of the immune response and the regulation of interactions of functionally distinct subpopulations of lymphocytes. Moreover, the results of these experiments will provide insights into the nature of aberrant lymphocyte responses and growth, including the decreased production of and response to various mediators in immunodeficiency diseases, and abnormal production and use of growth factors by tumor cells. (LB)